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374 METALLURGY OF CAST IRON.
I state the time between the pourings to give an idea of how long the metal was held.
The fluidity strips are the practical guide to go by. Of what use is time in regulating or asserting the fluidity of irons between two foundries, or one heat from another? The iron in no two foundries is of the same fluidity, or for that matter the same foundry will seldom have two days' run-in succession alike, and where one shop could only hold its metal for five minutes, another might do so for ten. There is no guide 'to register the fluidity of molten metal better than fluidity strips attached to test bars, as advocated by the author in Chapter LXVIL For scientific research and close regulating of mixtures by physical tests, it is essential for fluidity strips to be attached to test bars, where one desires to obtain true knowledge of irons or mixtures. I have shown that degrees in fluidity affect the depth of chill, also that it is incorrect for a test bar to pull away from its chill when contracting, as seen in Chapter LVL This latter evil only aggravates more the one caused by different degrees in fluidity, as both elements are effective in causing erratic depths of chill.
I could have shown a much more radical difference in the chill obtained from the same ladle by different degrees of fluidity, and would here say that in one case I found with the same iron in pouring two J^-inch bars that the dull-poured one had a chill of T\ inch, the hot-poured one j£ inch, a difference of •£$ inch.
For any that desire to test the question of degrees of fluidity causing different thicknesses in chill, m /-£-inch square test bars, I have presented the plan I used .in my experimenting with a J^-inch square test bar,s refers to iron possessing from 2.50 to 3.00 of silicon. For results of higher silicon, see next paragraph.ron, Ferrochrom.